Asked why he robbed more than 100 banks, the legendary Willie Sutton supposedly replied: “Go where the money is . . . and go there often.” In the wake of the debt-ceiling deal, the worry now is that Congress and President Obama are going to treat the US defense budget the way Sutton treated banks.

Considering the world today, that would be a big mistake.

Initially, the debt deal requires slashing defense some $350 billion over the next 10 years. And unless Democrats and Republicans can agree to other savings elsewhere in the budget by the end of the year, it could “trigger” a cut in Pentagon spending of up to $1 trillion in total.

The first problem is that war is a come-as-you-are affair. Just weeks shy of the 10th anniversary of the 9/11 attacks, we shouldn’t need reminding that there will always be unpleasant strategic surprises affecting our national security.

The operational readiness of our armed forces is already at risk. Experts assert that units across the four services in some cases aren’t “fully mission-capable” -- meaning they aren’t ready to fight tonight.

We may also not have enough of what we need. Did you know the US Navy now has fewer ships today than at any time since World War I? Or that the pilots who fly our strategic bombers (e.g., the B-52) are often decades younger than the planes themselves?

Besides the Constitution’s requirement for the Congress to provide for our national defense, it’s just wrong to ask our brave young men and women to go in harm’s way without the best equipment.

Added to all of this, there’s sure plenty for our military to worry about besides the fights we’re already in.

Even as we cut, China’s unprecedented military buildup shows no signs of abating. It will soon put an aircraft carrier to sea for its maiden voyage for what Beijing calls some “research and training.” The ship, along with other flattops being built, are just part of China’s increasingly apparent drive to control the Western Pacific -- and deny the United States access to it.

Elsewhere in Asia, North Korea remains a menace. Not only is the huge Korean People’s Army ready to strike into South Korea, it’s now reportedly developing road-mobile ICBMs to augment its long-range missile and nuclear arsenal.

In the Middle East, Iran is in the ascent with many of its former rivals reeling from Arab Spring revolutions. There’s been no progress in curbing the mullahs’ regional meddling or their atomic aspirations.

For years, Iran has been arming Iraqi militants and the Taliban and sharing strategic secrets with North Korea. Tehran is now feeling its oats enough to reportedly be aiding al Qaeda’s efforts in Afghanistan -- not to mention sending ships to ply the waters of the Mediterranean and, perhaps in the future, the Atlantic.

Oh, and it will likely have an ICBM by 2015. Talk about a great reason for missile defense, a vital program that may end up on the budget chopping-block if we’re not careful.

Sure, Osama bin Laden has been silenced for good, but we haven’t seen the last of al Qaeda. It’s losing ground in Afghanistan and under pressure in the Pakistani tribal areas, but seems to be doing quite well in such chaotic places as Yemen and Somalia.

The challenges don’t end there. We have good reason to worry about Russia, Pakistan and Venezuela, among others. Fact is, there’s no shortage of threats to our national security that we need to be ready to defend against.

It’s an increasingly dangerous world. We need a strong military that can protect the homeland, deter aggression as well as project power globally to protect our interests abroad if needed.

A Willie Sutton-like heist of the Pentagon budget will leave the United States forever reacting (as best we can) to world events -- because we’ll be unable to shape the security environment. That’s a sure way to guarantee the 21st century isn’t one of American primacy.

Peter Brookes is a Heritage Foundation senior fellow and a former deputy assistant secretary of defense.